In a wireless communication system, the power amplifier of a wireless transmitter amplifies a radio frequency signal (also referred as a high-frequency signal or a microwave signal) to a high power level, and transmits the amplified radio frequency signal to a wireless receiver through an antenna and a transmission medium (such as air).
The power amplifier is used in many fields, such as entertainment (remote control vehicle, unmanned aerial vehicle, air shot machine), global system for mobile communications (GSM), wideband code division multiple access (W-CDMA), long term evolution (LTE), wireless local area network (WLAN), military applications and space applications. In terms of power consumption, the power amplifier ranks number one or number two in the wireless communication system. In the wireless communication system, the power amplifier may have poor linearity, and may cause distortion or even jeopardize the quality and accuracy of signal communication.
During signal amplifying performed by the power amplifier, the third-order distorted signals near the main signal may deteriorate the quality of communication signals. FIG. 1 is a schematic diagram showing signal distortion. As indicated in the left-hand diagram of FIG. 1, if the main frequencies used by two users are far from each other, no or little interference will occur between the two main frequencies. As indicated in the right-hand diagram of FIG. 1, if the frequencies used by the two users are close to each other, the third-order distorted signals will interfere with each other and deteriorate the communication quality.
To maintain communication quality, to smoothly demodulate the signals at the receiver and to prevent interference on other users during signal transmission, signal transmission standards for the transmitter are defined. Adjacent channel leakage ratio (ACLR) may be used to measure the spectrum interference on users outside the channel when the communication signals are distorted. At a frequency away from the center frequency, the spectrum of the transmitter is required to be lower than a spectral mask. Thus, the power amplifier of the transmitter is required to have high linearity to prevent the spectral regrowth and to prevent the signal interference from adjacent channels.